The present invention relates to an improved toner for use in an electrostatic copying process. Such processes are now commonly used by laser printers and photocopy machines.
Electrostatic processes use finely divided dry toners or developers. Dry toners typically have a coloring agent, such as carbon black, dispersed throughout a natural or synthetic binder resin. Developers can be monocomponent substances in which the toner formula contains magnetic particles, monocomponent substances in which the toner is non- magnetic, or two component systems which have dry toners and separate magnetic carrier particles.
Toner particles can be triboelectrically charged to a proper polarity. This insures that the toner particles will be preferentially attracted to desired image areas on a latent image field during the copying or printing process. Often toners include charge control agents which moderate the magnitude and polarity of the charge. After the image is developed by the toner, the image on the latent image field is "fixed" onto a permanent substrate, such as paper. "Fixing" can be effected by applying heat or solvent to the toner image after the image has been transferred to a sheet of paper, which melts the resin particles in the toner, causing them to fuse to the paper or other substrate.
"Melt fixing," that is, fixing by applying heat to the toner image, is accomplished by applying heat to the sheet of paper by means of heated rollers. The rollers, known as fusing rollers, apply both heat and pressure to the sheet. Fusing rollers are widely employed because high-speed fixation of latent images can be achieved with this method.
In order to reduce fixing time, it is desirable for a binding resin to exhibit high fixing ability even at low temperatures. To achieve this end a binder resin having a low softening temperature typically is used. This practice, however, has the drawback that toner particles may partially stick to the fusing rollers, damaging them. Also, residual toner on the fusing rollers is transferred to subsequent sheets of paper. This stains the background of those sheets, resulting in poor image quality. This is the well-known and undesirable "offset" phenomena.
To retain the high fixing ability of these resins at low fixing temperatures while reducing the offset of toner, lubricants have been added to various systems in order to prohibit the toner particles from sticking to the fusing rollers. An initial effort was to provide fusing rollers covered with a thin film of lubricant, such as a fluorocarbon resin or silicone oil. This method, however, suffers from several drawbacks: for example, the heat from the fuser rollers can cause odors to emanate from the liquid or the roller coating, damage to the fuser rollers can result in a messy oil spill, and the use of silicone liquid requires a complex apparatus for metering of liquid to the rollers.
Another method which has been used to prevent or minimize this problem is adding a wax, such as a paraffin or polyolefin wax, to the toner itself. In theory, the wax component should be sufficiently incompatible with the toner resin so that it does not mix with the toner resin. This ensures that when the paper imprinted with the toner image passes through the hot fuser rollers, the wax rises to the surface of the toner, forming a waxy coating which prevents the toner from sticking to the rollers. However, paraffin or polyolefin waxes may adhere to the roller or paper surface, which reduces the quality of the image. In some toner compositions, the polyolefin wax is sufficiently compatible with the toner that it does not rise to the surface of the toner and, so, does not prevent offset from occurring. Various alcohols, varnishes and silicone oils have also been tried, but none have imparted a sufficiently high resistance to the offset phenomena. In addition, these lubricants showed a tendency to adhere to the carrier particles, reducing developer life and image quality.
The present invention achieves the aforementioned desirable characteristics while avoiding the undesirable characteristics of prior art toners.